HF voltage

Measurement of HF voltage restricted to coaxial lines and is reproducible only if the reference plane of the voltage measurement is well defined with within smallest fractions of a wavelength. Therefore, the upper frequency limit is at a few GHz. As in case of HF power measurements, the PTB's standard measuring method is based on a d.c. substitution method. The unknown HF voltage which, at an ohmic resistor, causes a temperature rise which is proportional to the real power, is replaced by a d.c. voltage of the same real power so that the rms-value of the HF voltage is measured.
The HF voltage standard is a d.c. coupled HF power sensor with frequency-dependent input conductance determined by calibration and with frequency-dependent effective efficiency measured in the microcalorimeter. The voltmeter to be calibrated is connected in parallel with this HF voltage standard, using a T-junction. The reference plane for the voltage measurement is the connector front plane at the sensor input (reference plane for the admittance measurement). Special electrically short T-junctions have been developed for both different transmission line and connector systems. The distance between the plane at which the standard and device under test are connected, and the symmetry plane of the junction (where the HF signal is coupled in) are very small compared with the wavelength. Measurement uncertainties due to the voltage standing wave are eliminated by this up to the upper frequency limit of 2 GHz.